Natural gases and liquefied petroleum gases are widely used as fuels for domestic, commercial and industrial purposes, e.g., for heating and/or cooking. As a result, there is often a danger that leakage of such gas from piping and other apparatus will contaminate the surrounding environment, creating a dangerous condition. For example, alkane gases such as methane are extremely combustible and furthermore could poison individuals if present at too great a level in enclosed surroundings. Moreover, small quantities of carbon monoxide can escape from such flowing gas streams into the environment, or could be generated by incomplete combustion of natural gas. Carbon monoxide is odorless and colorless, so that the contaminating levels are not readily observable by individuals. However, carbon monoxide is absorbed by an individual's lungs and reacts with the hemoglobin in the blood to form carboxyhemoglobin (COHb), reducing oxygen carrying capacity of the blood. Therefore, presence of carbon monoxide in an environment above certain levels is extremely dangerous and can easily poison individuals unaware of its presence.
Accordingly, there is a need to provide for suitable detection and concomitant alarm of unwanted fluid, i.e., gaseous contaminants in order to prevent unsafe or dangerous conditions from developing, e.g., in an enclosed environment. In this regard, various detectors for carbon monoxide and alkanes such as methane gas have been developed.
Various residential carbon monoxide gas detectors are found in the prior art and typically function to sound an audible alarm when a specified environmental CO concentration is detected. Carbon monoxide sensing elements employed within such detectors typically comprise a metal oxide layer on a ceramic substrate. When the ceramic substrate is heated to a high temperature, e.g., 200-400.degree. C., the resistance of the metal oxide varies as a function of the environmental CO concentration. The sensing element can thus be employed as a variable resistor in a calibrated detection/alarm circuit which continuously monitors the environmental CO concentration and sounds an alarm upon the detection of hazardous levels.
Methane detectors which employ metal oxide semiconductor sensors are also in the prior art. As with CO sensors, many methane sensors also operate by maintaining the ceramic substrate at a high temperature such that the resistance of the metal oxide predictably changes as a function of ambient CH.sub.4 concentration. Methane sensors are available which are relatively insensitive to the presence of carbon monoxide, and vice versa.
False alarms and/or gas detection inaccuracies due to the presence of airborne contaminants are often a problem with CO and CH.sub.4 detectors. The sensing elements are typically sensitive to the presence of airborne gases and vapors such as alcohols, solvents and water vapor. In the common household environment, the use of cleaning agents, paints, turpentine, solvents, etc., produce vapors which can alter the resistance of the sensing element, thereby causing false alarms in the detection of the target gas.
When carbon monoxide enters a person's bloodstream, it reacts with hemoglobin (Hb) to form carboxyhemoglobin (COHb). A person is in danger of carbon monoxide poisoning when the person's COHb level exceeds a specific level, such as the levels promulgated by Underwriters Laboratories UL2034 carbon monoxide exposure specification. An individual's COHb level is a function of their exposure time to the environmental CO concentrations. False alarms are particularly a problem in prior art CO sensors because these sensors do not accurately model the COHb level that would be present in the exposed person's bloodstream. For example, a relatively high environmental CO concentration that is present for only a very short time duration often causes an alarm to be sounded in prior art CO sensors when such exposure would actually produce a COHb level well below the danger level.
U.S. Pat. No. 4,896,143 to Dolnick et al. discloses a CO sensor which is operatively connected to a microprocessor for calculating doses of carbon monoxide. The values of CO are accumulated over time, with the dose measurement being made by adding a measured value of CO concentration, which may be weighted, to a memory register each time the value is determined. An alarm is issued if the accumulated value reaches a predetermined value. A drawback of this technique is that the physiological response is not accurately modeled, which may lead to false alarms and/or under-detection of CO.